This invention concerns presses, particularly presses used in thermoforming operations. Such presses include movable platens which mount forming dies, the dies overlain with one or more sheets of plastic to be formed into a part by the application of fluid pressure. The fluid pressure expands the central portions of the heated sheets into conformity with the contours of die cavities. The perimeter of the sheet is engaged by surfaces of the dies brought into contact with the sheet after the platens are moved together.
The platens are each driven by drive mechanisms to a closed position where there is a minimum separation distance between the platens, referred to as a "shut height" of the press. It is necessary to adjust the shut height position of the platens for different part applications. Where the drive mechanism has a maximum extended position which cannot be varied, as in a crank drive, some other means must be provided for adjustment of the shut height.
Wedge adjustments have been provided, in which tapered pieces are interposed between the die and the platen, as described in U.S. Pat. Nos. 3,889,515 and 3,489,049.
Wedge slides have also been used in combination with a linkage drive for the platens, as described in U.S. Pat. No. 4,133,260.
See also U.S. Pat. No. 5,814,185 issued to the assignee of the present application which uses threaded shafts and chain driven nuts to adjust the closed platen position.
Wedge structures associated with the dies are not practical with very large dies due to the difficulties in precisely maintaining parallelism between the upper and lower dies.
Another disadvantage of many of the wedge arrangements is that shut down of the machine is necessary to make adjustments.
Many platen drives utilize linkage systems which must be reversed after reaching a dead center position, which drive reversal increases the press cycling time and complicates the controls.
It has been proposed to utilize a unidirectional crank drive for thermoforming presses. However, the use of eccentric or crank drives inherently creates an overhang condition of the drive shaft used to rotate a crank plate. Very large loads are typically generated in thermoforming presses since the fluid pressure used to form the part is exerted over a large area, and the resulting large forces must be withstood by the shafts driving the crank plates. Since the shafts are necessarily cantilevered out from the drive unit, excessive bending of the cantilevered shaft is likely to occur.
It is an object of the present invention to provide a shut height adjustment arrangement and method for a forming press which maintains parallelism even between very large dies mounted to the platens, and which allows adjustments to be made when the machine is in operation.
It is a further object of the present invention to provide a crank drive arrangement for a thermoforming press which allows a non-reversing platen drive, but which does not result in excessive shaft bending by the large forces involved.